The term Giga Coaster was coined by Cedar Fair and Intamin AG to describe their world record breaking ride: Millenium Force. This coaster was bult in May 2000 and was the first coaster to go over 300 feet in height. Essentially, any coaster between 300 feet and 399 feet should be classed as a Giga Coaster.
The Original
Intamin Gigacoaster
The first (and only for ten years) Gigacoaster by Intamin AG was multi-award winning Millenium Force as Cedar Point. Millenium Force proved that you could have massive height coupled with an excellent layout people adore.
In 2010, Intamin's second Gigcoaster, Intimidator 305 was opened.
Variations
Chance-Morgan Giga Coaster
Within months of Millenium Force opening, Nagashima Spaland opened Steel Dragon 2000. This Chance-Morgan beast topped Millenium Force by an entire 8 feet and captured the record for height; not that it would be keeping it too long.